Five experiments are proposed to examine potential ontogenetic differences in the effects of stimulus preexposure and retention interval on conditioned aversions in rats. These experiments will analyze a recently reported interaction between stimulus preexposure and retention interval that is of special significance for contemporary theories of conditioning (Kraemer & Roberts, 1984). The experiments will test the generality of the preexposure-retention interval relationship with respect to temporal parameters, stimulus similarity, task specificity and age. Experiment 1 will examine the effects of flavor retention intervals of 1,7,14 and 21 days, and the effects of imposing these same intervals between preexposure and conditioning. These manipulations will determine the temporal and ontogenetic generality of the preexposure-retention interval interaction. Experiment 2 will assess the generality of the preexposure- retention interval relationship by testing with a conditioning procedure unlike the (possibly unique) taste aversion conditioning. Both preweanlings and adults will be tested, which will again test predictable age-related differences in the effects of preexposure and in the retention of odor aversions. Experiment 3 will examine the influence of stimulus similarity on the preexposure-retention interval relationship. Conditioned taste aversions in weanling and adult rats will be assessed at 1- and 21-day intervals, following exposure to flavors of varying degrees of similarity to the flavor CS. Experiment 4 will examine the effects of drug (US) preexposure on conditioned taste aversions in preweanlings and adults tested after 1- and 21-day retention intervals. This will provide a further test of the generality of the preexposure- retention interval interaction, and also determine whether age- related differences exist in the effects of drug preexposure. Finally, Experiment 5 will explore an intriging age-related difference in conditioned taste aversions found in our preliminary research. We found that chocolate milk aversions in preweanlings were significantly stronger when tested after a 21-day retention interval than when tested after a 1-day interval. Several types of evidence suggest that this effect may be yet another instance of the preexposure-retention interval interaction. Perhaps weanlings generalize their experience with mother's milk to the chocolate milk CS, producing relatively weak aversions when tested immediately, but stronger aversions when testing is delayed. Experiment 5 will explore this possibility.